Web sites and other types of interactive systems may implement recommendation services or a recommendation engine for recommending items stored or represented in a data repository. These services can operate, for example, by receiving as input a target item, and by outputting a ranked list of items that are collectively similar or related to the target item.
One common application for recommendation services involves recommending products for purchase, rental, subscription, viewing or some other form of consumption. For example, e-commerce web sites commonly provide services for recommending products to users based on their respective purchase histories, rental histories, product viewing histories, and/or item ratings. Recommendation services are also commonly used to recommend web sites, articles, users, music and video files, and other types of items.